Some engines, known as variable displacement engines (VDE), may be configured to operate with a variable number of active and deactivated cylinders to increase fuel economy. Therein, a portion of the engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including engine temperature. An engine control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve actuators that affect the operation of each cylinder's intake and exhaust valves. By deactivating engine cylinders at low speeds/light loads, associated pumping losses may be minimized, and engine efficiency may be increased.
In some instances, the mechanisms that actuate the deactivatable cylinder valves (e.g., VDE mechanisms, or VDE actuators) may degrade, leaving the intake and/or exhaust valves operating as though the cylinder was still active. As such, there is an increased propensity for exhaust valve degradation relative to intake valve degradation due to carbon from the exhaust gas depositing on the exhaust valve. In this situation, fuel economy may be impacted as the inability to seal the cylinder during deactivation results in pumping losses. Drivability may also be adversely impacted as unaccounted air or vapor may be directed through the catalyst from the leaky cylinder. This issue may be addressed by monitoring VDE mechanism functionality and timely identifying and addressing VDE degradation.
Various approaches have been identified for diagnosing degradation of VDE operation, such as diagnostic methods based on crankshaft vibrations related to engine firing order, firing frequency, measuring manifold pressure, etc. One example approach is shown by Doering et al. in U.S. Pat. No. 8,667,835, where indication of intake and/or exhaust valve degradation in a VDE engine is based on an estimation of manifold pressure over a plurality of immediately successive induction events. The manifold pressure response during the intake stroke of each cylinder may be monitored and an average change in manifold pressure in a defined sampling window of an intake stroke may be used to identify degradation in valve activation/deactivation mechanisms.
However, the inventors herein have recognized several disadvantages with such approaches. As an example, such approaches may be computationally intensive, requiring a plurality of MAP measurements and extensive data manipulation to perform the VDE system diagnostic while the engine is running. As another example, such approaches may not be able to distinguish between a cylinder with a portion of the cylinder valves functionally degraded and a cylinder with all of the cylinder valves functionally degraded. In yet another example, additional sensors may be required to monitor certain engine parameters in order to diagnose degradation of the VDE mechanisms, leading to increased cost. Further still, the approach may require engine operation in a VDE mode which may be limited during strictly city driving or during engine operating under heavy loads. Due to the VDE mechanisms not being actuated regularly, opportunities for diagnosing VDE degradation may be limited.
In one example, the issues described above may be at least partly addressed by an engine method comprising: responsive to a request to diagnose a cylinder valve actuator during a non-fueling condition of the engine, spinning the engine, unfueled, with all cylinders activated to determine a reference air flow amount, and then, selectively deactivating one or more cylinder valves, and indicating cylinder valve actuator degradation based on an air flow amount following the deactivating relative to a threshold, the threshold based on the reference air flow amount. In this way, degradation of the VDE mechanism may be detected using less computation and while relying on existing sensors.
As one example, during engine non-combusting conditions, such as after vehicle key-off, the engine may be cranked unfueled with the VDE mechanism of all cylinders activated. A delta pressure across an exhaust particulate filter (PF), which is indicative of air flow through an exhaust passage (herein also referred to as exhaust air flow), may be monitored via a delta pressure sensor coupled to the PF. Once the exhaust air flow reaches a steady state, one or more cylinders of the VDE may be selectively deactivated via their respective VDE mechanism. The one or more cylinders may be concurrently deactivated or each deactivatable cylinder in the VDE may be sequentially deactivated. Due to deactivation of one or more cylinders, there may be a corresponding decrease in exhaust air flow which may change the delta pressure across the PF. If after deactivation of the one or more cylinders it is determined that the delta pressure has not changed appreciably, a degradation in the VDE mechanism may be indicated and a diagnostic code may be set. Further, if after deactivation of one or more cylinders it is determined that the delta pressure has changed but continues to remain above a threshold delta pressure, it may be inferred that there is a partial degradation of the VDE mechanism (such as a leak in one of the cylinder valves) causing airflow through the cylinder(s) even during cylinder deactivation. Upon detection of VDE mechanism degradation, the engine may be operated with all cylinders active for at least the immediately subsequent engine cycle.
In this way, changes in air flow through an exhaust passage may be correlated with valve events to diagnose a VDE mechanism. By leveraging an existing delta pressure sensor to detect degradation of a VDE mechanism, the cost associated with the diagnostic may be reduced. By diagnosing the VDE mechanism during vehicle key-off, VDE health monitoring may be carried out opportunistically without having to wait for VDE conditions to be met. The technical effect of evaluating the VDE system during an engine unfueled condition with minimal data collection is that diagnostics may be performed independent of an operator's driving habits and without affecting drivability. Also, by comparing delta pressure across a PF during VDE and non-VDE modes, potential degradation of the VDE mechanisms may be assessed without extensive computational requirements.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.